1. Field of the Invention
The present invention is directed to a catalyst system to facilitate the reduction of nitrogen oxides (NOx) and ammonia from an exhaust gas. More particularly, the catalyst system of this invention includes a lean NOx trap in combination with an ammonia selective catalytic reduction (NH3-SCR) catalyst, which stores the ammonia formed in the lean NOx trap during rich air/fuel operation and then reacts the stored ammonia with nitrogen oxides to improve NOx conversion to nitrogen when the engine is operated under lean air/fuel ratios. In an alternate embodiment, a three-way catalyst is designed to produce desirable NH3 emissions at stoichiometric conditions and thus reduce NOx and NH3 emissions.
2. Background Art
Catalysts have long been used in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into non-polluting gases such as carbon dioxide, water and nitrogen. As a result of increasingly stringent fuel economy and emissions standards for car and truck applications, it is preferable to operate an engine under lean conditions to improve vehicle fuel efficiency and lower CO2 emissions. Lean conditions have air/fuel ratios greater than the stoichiometric ratio (an air/fuel ratio of 14.6), typically air/fuel ratios greater than 15. While lean operation improves fuel economy, operating under lean conditions increases the difficulty in treating some polluting gases, especially NOx.
Regarding NOx reduction for diesel and lean burn gasoline engines in particular, lean NOx adsorber (trap) technologies have been widely used to reduce exhaust gas NOx emissions. Lean NOx adsorbers operate in a cyclic fashion of lean and rich durations. The lean NOx trap functions by adsorbing NOx when the engine is running under lean conditions-until the NOx trap reaches the effective storage limit-followed by NOx reduction when the engine is running under rich conditions. Alternatively, NOx reduction can proceed by simply injecting into the exhaust a sufficient amount of reductant that is independent of the engine operation. During this rich cycle, a short rich pulse of reductants, carbon monoxide, hydrogen and hydrocarbons reduces the NOx adsorbed by the trap during the lean cycle. The reduction caused during the rich cycle purges the lean NOx adsorber, and the lean NOx adsorber is then immediately available for the next lean NOx storage/rich NOx reduction cycle. In general, poor NOx reduction is observed if the air excess ratio λ is above 1. NOx reduction generally increases over lean NOx adsorbers as the λ ratio is decreased lower than 1. This air excess or lambda ratio is defined as the actual air/fuel ratio divided by the stoichiometric air/fuel ratio of the fuel used. The use of lean NOx adsorber (trap) technology, and in particular the rich pulse of reductants, can cause the λ ratio to reach well below 1.
Lean NOx traps, however, often have the problem of low NOx conversion; that is, a high percentage of the NOx slips through the trap as NOx. NOx slip can occur either during the lean portion of the cycle or during the rich portion. The lean NOx slip is often called “NOx breakthrough.” It occurs during extended lean operation and is related to saturation of the NOx trap capacity. The rich NOx slip is often called a “NOx spike.” It occurs during the short period in which the NOx trap transitions from lean to rich and is related to the release of stored NOx without reduction. Test results depicted in FIG. 1a have shown that during this lean-rich transition, NOx spikes, the large peaks of unreacted NOx accounts for approximately 73% of the total NOx emitted during the operation of a lean NOx trap. NOx breakthrough accounts for the remaining 27% of the NOx emitted.
An additional problem with lean NOx traps arises as a result of the generation of ammonia by the lean NOx trap. As depicted in FIG. 1b, ammonia is emitted into the atmosphere during rich pulses of the lean NOx adsorber. In laboratory reactor experiments, ammonia spikes as high as 600 ppm have been observed under typical lean NOx adsorber operation (see FIG. 1b). While ammonia is currently not regulated, ammonia emissions are being closely monitored by the U.S. Environmental Protection Agency; and, therefore, reduction efforts must be underway. Ammonia is created when hydrogen or hydrogen bound to hydrocarbons reacts with NOx over a precious metal, such as platinum. The potential for ammonia generation increases for a precious metal catalyst (such as a lean NOx trap) as the λ ratio is decreased, as the duration of the rich pulse increases, and the temperature is decreased. There is thus an optimum λ and rich pulse duration where the maximum NOx reduction is observed without producing ammonia. Attempts to enhance conversion of NOx by decreasing the λ ratio of the rich pulse duration leads to significant production of ammonia and thus results in high gross NOx conversion (NOx→N2→NH3), but much lower net NOx conversion (NOx→N2).
In addition to nitrogen, a desirable non-polluting gas, and the undesirable NH3 described above, N2O is another NOx reduction products. Like NH3, N2O is generated over NOx adsorbers and emitted into the atmosphere during rich pulses. The gross NOx conversion is the percent of NOx that is reduced to N2, N2O and N3. The net NOx conversion is the percent of NOx that is reduced to nitrogen, N2, only. Accordingly, the gross NOx conversion is equal to the net NOx conversion if nitrogen is the only reaction product. However, the net NOx conversion is almost always lower than the gross NOx conversion. Accordingly, a high gross NOx conversion does not completely correlate with the high portion of NOx that is reduced to nitrogen.
The NOx conversion problem is magnified for diesel vehicles, which require more than a 90% NOx conversion rate under the 2007 U.S. Tier II BIN 5 emissions standards at temperatures as low as 200° C. While high NOx activity is possible at 200° C., it requires extreme measures such as shortening the lean time, lengthening the rich purge time, and invoking very rich air/fuel ratios. All three of these measures, however, result in the increased formation of NOx or ammonia. Accordingly, while it may be possible to achieve 90+% gross NOx conversion at 200° C., to date there has not been a viable solution to achieve 90+% net NOx conversion.
Accordingly, a need exists for a catalyst system that eliminates NOx breakthrough during the lean operation as well has the NOx spikes during the lean-rich transition period. There is also a need for a catalyst system that is capable of improving net NOx conversion. Finally, there is a need for a catalyst system capable of reducing ammonia emissions.